The full phrase seems to be an attempt at a mixed-language question: "Shinseki no ko to o tomari dakara… que es español?" — possibly meaning: "Because I'm staying over with a relative's child… what does that mean in Spanish?"
That night, they mapped the journey from Tokyo to Madrid on a napkin. Sofía taught Haru how to say “Estoy en casa” — “I’m home” — and Haru taught her “Okaeri” — “welcome back.” shinseki no ko to o tomari dakara que es espa%C3%B1ol
The lyrics of the chorus go:
If you are looking for the specific media title associated with this phrase, it is likely an adult-oriented Doujinshi (self-published work) or an anime episode titled something akin to "Staying with my Cousin." The full phrase seems to be an attempt